Nitrogen (N) is an essential element for the growth and yield of rice. Some endophytic bacteria can fix N 2 from the air and convert to nitrogen compounds that can be utilized by plants. In this study, endophytic bacteria were isolated from one-month-old seedlings of five rice (Oryza sativa L.) varieties (Muey Nong 24, Muey Nong 25, Pathum Thani 1, Suphan Buri 1 and Chai Nat 1) growing without nitrogen fertilizer in the farmers' field. One hundred and twenty-three isolates of endophytic bacteria were obtained from the roots, stems and leaves of these rice varieties. Nitrogenase activity of the bacteria in N-free culture medium was determined by acetylene reduction assay. Seven isolates of the bacteria with highest nitrogenase activity were identified by phylogenetic analysis of the 16S rRNA genes, and found to belong to Burkholderia cepacia (CS5), Citrobacter sp.